The field of the present invention is lash adjusters for valve trains on internal combustion engines.
Generally, lash adjusters have been known for zero-adjusting a gap or clearance, such as a gap between a valve and a cam in a valve train of an internal combustion engine. A known example of such a lash adjuster is the so-called sealed-type hydraulic lash adjuster which comprises a tubular body having one end thereof closed by an end wall. A plunger slidably fitted into the body cooperates with the inner surface of the end wall of the body to form a high pressure chamber containing a hydraulic oil. A plunger spring is interposed between the plunger and the body to resiliently urge the plunger out of the body. A flexible diaphragm is mounted between the open end of the body and the plunger to define an annular storage chamber for the hydraulic oil to be supplied to the high pressure chamber. A check valve is provided in the high pressure chamber to prevent the hydraulic oil flowing from the high pressure chamber to the storage chamber.
In use, the sealed-type hydraulic lash adjuster may be mounted to a rocking center of a rocker arm disposed between a cam and a valve. If a clearance develops between the rocker arm and the valve cam, the plunger is projected from the body by the plunger spring, causing the rocker arm to be moved toward the valve and cam, thereby removing the clearance. In addition, as the body and the plunger are moved in relation to each other, the hydraulic or working oil is supplied from the storage chamber to the high pressure chamber, through the check valve. The hydraulic oil transferred into the high pressure chamber is sealed therein by the check valve to prevent the plunger from being retracted into the body, so that any clearances in the valve train assembly, which may develop due to thermal expansion or wear, are continuously compensated for and removed. Moreover, the change in volume of the storage chamber due to the out flow of the hydraulic oil is compensated for by the flexibility of the diaphragm.
However, when a hydraulic lash adjuster (HLA) is used over a long period of time, the amount of hydraulic oil in the reservoir chamber can change due to migration of hydraulic oil from the adjuster or migration of lubricant to the adjuster from the engine. A decrease in the amount of hydraulic oil in the reservoir chamber will not significantly affect the operation of the HLA. On the other hand, an increase in the amount of hydraulic oil in the storage chamber increases the pressure in the chamber, and adversely affects the operation of the plunger.